I don't understand this reluctance to share an idea for a startup. It's ridiculous, because it seems to be predicated upon the assumptions that no one has thought of it, and if they did they would go off and immediately implement it. What arrogance. You could tell them every little detail of what you have and mind and, because language is so vague, they could think it was something entirely different.
If you tell me about your startup, your first goal, before you can make me a user or (even better) a paying user, is to get me to give a shit about it, which is a formidable task.